Electrophysiological research has identified two distinct neural systems in the primate visual pathway, called the tonic and phasic systems, which process visual information in fundamentally different ways. The tonic system appears to be specialized for spatial detail and color, whereas the phasic system is specialized for rapid temporal luminance changes. I propose a comprehensive psychophysical investigation of the roles of the tonic and phasic systems in human vision. Specific areas of the investigation will include: (1) relative contributions of the tonic and phasic systems in color vision, spatio-temporal vision, light and dark adaptation, sensitivity and brightness; (2) comparison of the tonic and phasic systems in rod and cone vision; (3) variations in properties of the tonic and phasic systems as functions of retinal locus. This research is expected to lead to the development of clinical tests designed to reveal selective defects of the tonic and/or phasic systems in abnormal vision.